Calibration device for use in calibrating a sensor

ABSTRACT

In one embodiment the calibration device (1) comprises a housing (2) which by means of a removable lid (8) and a rupturable barrier layer (3) is divided into a calibration chamber (4) and a second chamber (10). The calibration chamber (4) contains a calibration fluid (5)--e.g. a dry gas--while second chamber (10) contains a second fluid (11)--e.g. a wetting agent. Immediately prior to a calibration process the lid (8) is removed, whereafter a sensor with its measuring surface in front is inserted through the second chamber (10) into the housing (2) until it abuts against a shoulder (6) in the housing (2). This causes the barrier layer (3) to rupture and the second fluid (11) to contact the calibration fluid (5). Hereafter the calibration process can take place. In another embodiment the device (1) further comprises a membrane (12) which is accommodated in the second chamber (10) and is adapted to be secured to the sensor during the insertion of the sensor into the housing (2). The disclosed device--which is particularly suited for use in calibrating sensors for transcutaneous registering of the contents of CO 2  and/or O 2  in blood--is self-contained, and can be made in a very compact design, making it very well suited for use in connection with portable equipment. The device is also suitable for being manufactured in the form of a disposable unit.

This invention relates to a calibration device for use in calibrating asensor, and of the type comprising a calibration housing with acalibration chamber, said calibration chamber containing a fluid for usein the calibration process and further including a wall part adapted tocooperate with the sensor in such a way that during the calibrationprocess the requisite contact for this purpose is established betweenthe sensor and the fluid contained in the calibration chamber.

Sensors are utilized in a wide range of applications in which more orless frequently a measurement of the content of one or more componentsin a given fluid is desired, or where it is desired to establish ameasurement of the condition of such a fluid e.g. as expressed by thepH-value of the fluid.

The components in question may exist in a gaseous state or in anon-gaseous state and may be present in either a liquid or in a gas.

The following components may be stated as examples of non-gaseouscomponents the content of which it is often desired to register: Ions(such as H⁺, K⁺, Na⁺, Ca²⁺, Cu²⁺, Cl⁻ etc.) and organic molecules (e.g.glucose, metabolites, hormones, enzymes etc.). Examples of gaseouscomponents include various gases such as O₂, NH₃ and CO₂. Such gases aretypically registered in the form of their respective partial gaspressures (pO₂, pNH₃ and pCO₂). Examples of fluids in which the abovecomponents may be present are various physiological liquids (e.g blood),various types of water (e.g. fresh water, seawater, sewage water etc.)nutritive mediums, laboratory samples of all sorts, flue gases, air etc.

As to typical sensors these include sensors based on variouselectro-chemical measuring principles such as potentiometry, amperometryand polarography as well as optical sensors, i.e. sensors utilizing anoptical response phenomenon.

Common to the majority of these sensors is the fact that prior to ameasuring sequence the sensor with accompanying equipment must becalibrated.

Such a calibration typically involves contacting the sensor with a fluidhaving a known content of the component to which the sensor issensitive.

Thus, in performing the calibration process it is necessary to haveaccess to a calibration fluid of a known composition. Also it isadvantageous if the calibration fluid is available in a form in whichthe requisite contact with the sensor is readily obtained.

Even though it is sometimes necessary or preferred to prepare the actualcalibration fluid immediately before the calibration process, it isnormally preferred to make use of a prefabricated calibration fluid. Incase of such a prefabricated calibration fluid it goes without sayingthat it is of vital importance to store said fluid under such conditionsthat the original composition does not change with time e.g. as a resultof mixing or reaction with other materials or the surroundings.

In this connection it is noted that handling of calibration fluidsintended for use in calibrating gas sensors, in particular sensors forpO₂ and pCO₂, poses special problems due to the fact that the containedcomponents tend to interact with the surroundings. Consequently it isnecessary, both during storage and use of such fluids, to take specialprecautions in order to minimize or eliminate the risk of any suchunwanted interaction. The optimum solution to this problem is of courseto store the fluid in question in a sealed chamber until the time ofuse. In connection with the calibration process proper the best solutionis to ensure that the contact between the sensor and the calibrationfluid be established in a way which positively excludes any priorcontact between the calibration fluid and the surroundings.

Another demand which has become topical concurrent with the developmentof portable measuring equipment is, that the auxiliary equipment used inconnection with the portable measuring equipment proper, must not initself--due to its size, shape or weight--reduce the portability of thetotal equipment or complicate the use of same.

Since the calibration equipment is a central part of the total measuringequipment it goes without saying that said calibration equipment shouldalso, to the maximum extent possible, comply with the above mentioneddemand.

Present commercially available calibration equipment does not to anysignificant degree possess the quite considerable degree of portabilitypossessed by recent, very compact measuring units.

One such very compact measuring unit is the TCM3-monitor fortranscutaneous measuring of the O₂ and/or the CO₂ content in blood, saidunit being developed and marketed by the Danish company RADIOMETER A/S.Having external dimensions of ca. 24×8×23 cm and having a weight of 2.7kg this unit is truly portable. This portability is, however,considerably impaired by the fact that during calibration the unitdemands usage of a separate, relatively bulky calibration device.

Said calibration device--which by and large consists of a bottlecontaining pressurized calibration gas (O₂ and/or CO₂)--is to beconnected via a flexible hose to a calibration house on the monitor, thecalibration house also being adapted to accommodate the sensor. Thecalibration device is not designed for permanent securing to the monitorand the two units thus cannot be handled as one self-contained unit. Inall circumstances such a combined unit would possess a considerablylower degree of portability as compared to the TCM3-monitor proper, dueto the size and weight of the calibration device (ca. 12×8×23 cm, 1.9kg).

U.S. Pat. No. 4,635,467 (Hoffa et al.) discloses a calibration device asdescribed in the opening portion of the present description. Said devicecomprises a liquid filled tube which in either or both ends is delimitedby means of a fluid-tight, gas permeable membrane. A further membrane,also being fluid and permeable to gas, forms a wall part which isadapted to abut the measuring surface of the sensor during calibration.Until the time of use all three membranes are covered by separate,manually detachable lids.

In calibrating for gaseous components it is necessary to have access toan external source of calibration gas. This source--which necessarilywill be in the form of a separate unit--is to be connected to the gasinlet of the device upon removal of the lids of the device and securingof the latter to the sensor. Upon a completed calibration process thegas source is to be disconnected from the de3: vice.

Thus, the calibration device according to U.S. Pat. No. 4,635,467 is notconvenient in use. Also the need for a separate gas source entails thatthe device be less suited for use in connection with portable equipment.

Accordingly there exists a need for a compact calibration equipmentwhich is well suited for use in connection with portable measuringequipment, which is convenient to handle and which does not call forspecial precautions in connection with storage or use and which furtherprovides for the possibility of establishing optimum conditions duringcalibration.

The above referred need is met by the calibration device according tothis invention, said device being characteristic in that the calibrationchamber consists in a closed, fluid-tight chamber, that said wall partconsists in a fluid-tight, rupturable and preferably non-elastic barrierlayer, which is adapted to be ruptured by contact with the sensor, andin that the fluid in the calibration chamber is a calibration fluidcontaining all the components necessary for the calibration process.

Such a calibration device can be designed very simply thus making itparticularly suited for mass production in the form of a disposableunit, thereby achieving various efficiency related advantages. Thephysical shape of the calibration device depends alone on the shape ofthe sensor with which it is to be used and the device can be used inconnection with calibration of sensors for registering both gaseous andnon-gaseous components.

Besides, such a disposable device or unit allows for the possibility ofmaintaining the sensor in a calibrated, ready-to-use state for extendedperiods of time, without continuously consuming calibration fluid.

A further advantage of such a disposable calibration device is that itmay be utilized as a quality control device when filled with a fluid ofpredetermined composition. A quality control device is used to determineor test the reliability of measuring equipment. In this connection it isto be understood that the calibration device according to this inventionis not in any way limited to the above-mentioned two areas ofapplication but is applicable within a wide range of applications wherethe task is to contact a sensor with a predetermined fluid.

The fact that the calibration chamber is delimited by a rupturablebarrier layer provides for a considerable degree of user-friendliness inthat the requisite contact between the calibration fluid and the sensoris obtained simply by forcing the sensor through the barrier layer.

Further the above mentioned risk of "pollution" of the calibrationfluid--this risk being particularly distinct in connection with agaseous calibration fluid--is prevented in that the calibration fluid isprevented from interacting with the surroundings before or during thecalibration process. On completed calibration the sensor and thecalibration device are separated and the latter may be discarded.

In principle, a calibration process demands that only the componentsrelevant to the calibration be present in the calibration chamber. Yet,it is sometimes desirable to be able to supply a supplementary fluid tothe calibration chamber in order to make the calibration "atmosphere"contacting the sensor simulate the actual conditions on the measuringsite in question.

If, for example, the task is to register the presence of a gaseouscomponent, e.g. O₂, in a liquid , it is, with a view to optimizing theaccuracy of the measurement, advantageous to perform a so-called "wet"calibration process, i.e. a calibration utilizing a "wet" calibrationgas. This could simply be effected by using a wet calibration gas in thecalibration chamber. Such an approach will, however, often entailproblems in the form of inaccurate calibrations and thus erroneousmeasurements, due to the risk of unintended interaction between thechemically quite reactive O₂ and possible impurities or other foreigncomponents present in the calibration chamber. Any such interactionwould mean that the O₂ -value would slowly drift away from its basic,predetermined value.

Thus, in case a wet calibration is to be performed it is advantageous tostore the calibration gas in a "dry" state and not wet it until thecalibration process actually takes place.

Consequently a preferred embodiment of the disclosed calibration deviceis characterized in that the housing comprises a second chamber, saidsecond chamber being delimited by the housing, the side of the barrierlayer facing away from the calibration chamber and a lid extending byand large parallel to the barrier layer, and in that the second chambercontains a second fluid which contacts the calibration fluid of thecalibration chamber upon rupture of the barrier layer.

This provides in a simple manner for a positive separation of thecalibration fluid and the wetting agent. The wetting automaticallyoccurs when the sensor ruptures the barrier layer due to the thusestablished connection between the second chamber and the calibrationchamber. The lid which together with the housing delimits the secondchamber is advantageously manually removable, but may also take the formof a rupturable lid which is to be ruptured by the sensor, similar tothe barrier layer.

Some of the various types of sensors, in particular gas sensors, use asemi-permeable membrane allowing passage of--and thus registeringof--the actual components while excluding other components.

Depending on the type of membrane and the use of same, the membrane hasto be replaced at regular intervals. The optimum interval forreplacement would be in connection with each new measuring cycle i.e. inconnection with each calibration. In practice, however, such a frequentreplacement will rarely take place i.a. due to the fact that themembrane replacement procedure--depending on the technique used--may beof a more or less complicated nature and/or time consuming. In somecases where the replacement procedure is carried out by means of easyoperable, discardable devices, the reason for less frequent replacementsof the membrane may be found in the cost of such discardable devices.

In the light of the above discussed circumstances, a further embodimentof the calibration device according to the invention is characteristicin that a membrane, which is permeable to the components relevant to thecalibration process, is arranged on the side of the barrier layer facingaway from the calibration chamber, said membrane extending by and largeparallel to the barrier layer, and in that the device comprises fixingmeans being adapted to cooperate with the membrane and the sensor insuch a way that the membrane is secured to the sensor in a stretched outstate upon moving the sensor and the membrane against each other in adirection by and large perpendicular to the plane of the membrane.

By adding to the calibration device according to the invention such anintegral membrane it is in a simple manner made possible to replace themembrane and calibrate the sensor in one single procedure. This providesfor a considerably increased degree of user-friendliness.

The membrane proper may consist of a polymeric membrane, known per se,which on the side facing the sensor has fastening or fixing means in theform of a ring having an annular projection adapted to engage acomplementary recess on the sensor. Alternatively the securing of themembrane to the sensor may be effected by means of a separate,resiliently deformable fixing ring in the form of an "O-ring" or thelike, situated on the side of the membrane facing away from the sensor.Upon pressing the calibration device against the sensor said ring will"roll" the membrane onto the sensor and from the "outside" keep themembrane attached to the sensor.

Said principle--incidentally known from prior art application devices ofthe discardable type--possesses in comparison with sensors using anelectrolyte between the membrane and the measuring surface of the sensorthe advantage of a very high degree of certainty that no air bubbles betrapped in the electrolyte, this being of importance to the measuringaccuracy.

In connection with the latter fastening principle it may be necessarywith some sort of guiding surface to assist the application of thefixing ring. This surface may form a part of an abutment surfaceintegral with the housing, said abutment surface determining thedistance to which the sensor is able to extend into the housing, but mayalso take the form of a separate guiding surface.

The invention will now be explained in detail with reference to theaccompanying drawing in which FIG. 1 shows a section through a firstembodiment of a calibration device according to the invention, FIG. 2 asection through a second embodiment of the calibration device accordingto the invention, FIG. 3 a section through a third embodiment of thecalibration device according to the invention, FIG. 4 a viewillustrating the calibration device of FIG. 3 at the beginning of acombined membrane replacement and sensor calibration procedure, FIG. 5 aview, on a larger scale, of a part of the membrane ring, seen "frombelow", FIG. 6 a partial section, on a larger scale, of the calibrationdevice of FIG. 4 after the membrane replacement procedure, and during acalibration process, FIG. 7 a partial section, on a larger scale,through a fourth embodiment of the calibration device according to theinvention and with an alternative membrane arrangement, FIG. 8 a partialsection, on a larger scale, through a fifth embodiment of thecalibration device according to the invention, FIG. 9 a perspectiveview, on a larger scale, of another embodiment of a membrane ring, FIG.10 a partial section, on a larger scale, through a calibration deviceincorporating the membrane ring of FIG. 9 and depicting the elementsjust prior to a combined calibration and membrane replacement procedure,and FIG. 11 a partial section similar to the one shown in FIG. 9, butdepicting the elements during such a procedure.

The calibration device appearing in FIG. 1 consists of a calibrationhousing 2, in which is accommodate a barrier layer 3. Barrier layer 3together with housing 2 defines a closed calibration chamber 4 in whichthe actual calibration fluid 5 is placed. The housing is of by and largecircular cylindrical shape and is made of a material which isimpermeable to the actual fluid 5, which does not interact with thisfluid and which at the same time gives the housing the necessarystrength. Suitable materials for the housing includes various metalssuch as aluminum, copper, brass etc. but also glass and various types ofplastics could be used. In the latter case, it might, however, provenecessary--with a view to the tightness of the housing--to provide thehousing with an interior coating of a metallic material e.g. appliedelectrochemically. The housing proper may be manufactured by anyconvenient method e.g. by casting, machining or deep-drawing. At presentit is preferred to manufacture the housing by deep-drawing of analuminum foil having a thickness of about 0.15 mm.

The barrier layer also has to be tight against the actual fluid and ispreferably made from an aluminum foil of a thickness about 0.02-0.04 mm.

Both aluminum foils used may have a coating of a thin polymeric foil onat least the one side, this making it possible to bond the barrier layerto the housing in a simple manner by means of a welding process.

As shown housing 2 comprises an annular shoulder 6 located a certaindistance from the bottom of chamber 4. This shoulder serves to limit thedepth to which the sensor is able to enter into chamber 4. The barrierlayer is arranged a certain distance "above" this shoulder 6.

FIG. 2 depicts a calibration device identical to the one shown in FIG.1, but supplemented with a lid 8 which is attached to an annular flange9 extending along the rim of the housing. Lid 8 together with barrierlayer 3 and housing 2 defines a second chamber 10. Said second chamber10 is adapted to hold a second fluid 11 which in connection with thecalibration process is to contact calibration fluid 5 in chamber 4. Lid8 which normally will be made of the same material as the rest of thecalibration device is preferably removably fastened to said device. Lid8 may be either glued or welded to housing 2.

FIG. 2 further illustrates an advantageous form of barrier layer 3.Here, barrier layer 3 forms a sort of "pot-like" insert which is onlyfastened to housing 2 along flange 9, and the bottom of which extends inthe predetermined distance from shoulder 6.

The embodiment illustrated in FIG. 2 is advantageous e.g. when acalibration is to be performed by means of a wet gas. In this case thecalibration chamber contains a suitable wetting agent possibly in theform of an electrolyte, or in combination with an electrolyte.

For the sake of good order it is noted, that the wetting process initself may effect a certain, minor shift in the concentration of thecalibration fluid in the calibration chamber. It is, however, possibleto compensate therefor simply by adjusting the basic concentration ofthe calibration fluid with a view to the desired concentration duringcalibration.

In FIG. 3 the calibration device is further supplemented with a membrane12 accomodated in second chamber 10 between barrier layer 3 and lid 8.

The membrane is of a type known per se and consists of a thin, circularpolymeric foil which along its outer edge is secured to a relativelyrigid plastic ring having an annular, inwardly directed projection. Theouter diameter of the plastic ring is slightly less than the innerdiameter of second chamber 10, and the inner diameter of the ring isslightly larger than the diameter of calibration chamber 4. Due to theforce subsequently exerted by the sensor the ring will engage shoulder 6which thereby serves as a rest for the membrane ring during thefastening of said ring--and thus also the membrane--to the sensor.

FIG. 4 depicts the calibration device at the beginning of a combinedmembrane replacement procedure and calibration process. The calibrationdevice has been opened by removal of lid 8, and second chamber 10 isexposed with its content of wetting agent/electrolyte 11 and membrane12. A sensor 14 with a measuring front surface 15 is ready to beinserted into the second chamber in the direction indicated by thearrow.

After a certain forward travel the sensor abuts the membrane ring andmembrane 3 attached to this ring. If the sensor is advanced further inthe direction of the arrow the side of the membrane ring facing thebarrier layer will cause said layer to break. A point of importance inthis context is, that the ruptured barrier layer must not contact (e.g.adhere to) the measuring surface of the sensor during the subsequentcalibration process. In other words, the rupturing of the barrier layermust be effected in such a controlled or guided way that it is ensuredthat said layer does not impede or prevent free access between themembrane covered measuring surface of the sensor and the calibrationfluid.

Such a guided rupturing might be attained by means of a suitable designof the side of the membrane ring facing the barrier layer. If, forexample, the radially outer edge of the ring takes the form of a cuttingedge along approx. 3/4 of the circumference of the ring, and theremaining 1/4 has a smooth rounded contour, approx. 3/4 of thecircumference of the barrier layer will be ruptured along the inner wallof the housing while the remaining 1/4 will not be ruptured. Experimentshave shown that this will cause the barrier layer to bend away in a moreor less plane state, very similar to the lid of a tin.

FIG. 5 shows a membrane ring which by simple means is provided with theabove mentioned cutting properties. As shown, the ring has along itsouter edge a relatively large number of closely spaced indentations 18,giving the ring a saw-like profile with points 19. These points effectthe rupturing or cutting of the membrane layer along the desired portionof the circumference.

Alternatively the desired, guided rupturing of the barrier layer may beeffected by a suitable arrangement of score lines or the like in thebarrier layer proper.

Upon rupture of the barrier layer and after a certain travel the ringwill abut shoulder 6 and as a result of further advancement of thesensor the ring will positively engage the sensor. The resultingoutwardly directed deformation of the membrane ring will, due to theabutment against the inner side of housing 2, assist in providing thedesired and necessary sealing between calibration chamber 4 and thesurroundings.

FIG. 6 illustrates, on a larger scale, a detail of the interconnectionbetween the sensor and the calibration device during calibration. In thecase shown the sensor consists in a sensor for transcutaneousregistering of the CO₂ -content in blood, a so-calledSeveringhaus-electrode. The working of said electrode depends on itspH-sensitive glass-section being electrically connected to thereference-section of the electrode via a suitable electrolyte, e.g. abicarbonate solution. Accordingly the second fluid here consist of abicarbonate solution, while the calibration fluid consists of a gasmixture with a suitable, predetermined content of CO₂, e.g. a mixtureconsisting of 5% CO₂, 20,9% O₂ and 74,1% N₂.

With a view to minimize the stabilization period of time of the sensorit is advantageous to use a bicarbonate solution having a content of CO₂corresponding to a partial gas pressure value (pCO₂) in the range ofe.g. 40-80 mmHg preferably about 60 mmHg. Since this value roughlycorresponds to the pCO₂ value which the sensor will register during thesubsequent transcutaneous measuring procedure the stabilization periodof time of the sensor will be minimized. In the case of an O₂ sensor asuitable content of O₂ in the electrolyte in question would be of anorder of magnitude corresponding to a partial gas pressure reading (pO₂)in the range of 40-170 mmHg.

As seen, membrane ring 16 engages sensor 14, so that membrane 12 is heldin a stretched out state over measuring surface 15 of the sensor.Barrier layer 3 is ruptured and as previously explained bent away fromthe sensor and into the calibration chamber so that the calibration gasin the calibration chamber has free access to sensor 14. Some of thewetting liquid 11--in the form of an electrolyte liquid--which wasoriginally held in second chamber 10 has, upon rupture of the barrierlayer, contacted the calibration gas and effected the desired wetting ofsame. Some of liquid 11 is confined between measuring surface 15 ofsensor 14 and membrane 12 as desired.

The radially outer side of sensor 14 and the back side or outer side ofmembrane ring 16, as mentioned, is pressed tight against the inner sideof housing 2 which in connection with a film of electrolyte liquid alsopresent here, entails that no fluid exchange between the surroundingsand the calibration chamber can occur during calibration.

Even in case of a gaseous calibration fluid this sealing will normallybe sufficient due to the fact that the pressure of the confined gasadvantageously corresponds to the pressure of the surroundings.

This, however, does not exclude that in other cases it may be desirableor convenient to make use of supplementary sealing means between thesensor and the calibration device Such means could for example be asuitable, possibly separate O-ring.

In any circumstances it is to be ensured that the securing of themembrane ring to the sensor and said engagement between the outer sideof the membrane ring and the inner side of the housing is mutuallyadapted in such a way that there is no risk of the membrane ring"sticking" in housing 2 when the sensor is removed from the calibrationdevice upon completed calibration and/or membrane replacement.

As described above the interconnection between the sensor and thecalibration device in this embodiment relies solely on the frictionalengagement between the outer side of the sensor and the inner side ofthe housing of the calibration device.

To further ensure the mutual interconnection the housing and/or thesensor could be provided with various connection assisting means, suchas projections, constrictions, spring locks etc. It is, however, alsopossible to envisage the necessary connection be effected by means of apurpose-built fixture either in the form of a separate tool or integralwith the measuring equipment in question.

FIG. 7 shows an alternative embodiment of membrane 12 with its fixingring. This embodiment differs from the above described one in thatmembrane ring 16 is not secured to the membrane proper, and in that saidring is located on the other side of the membrane, i.e. in between themembrane and barrier layer 3. Membrane ring 16 proper is not altered butcould have any suitable cross-section, e.g. a circular cross-section.Membrane 12 is positioned in the housing by attachment to flange 9 ofsaid housing. In case the sensor in question demands the presence of anelectrolyte between its measuring surface and the membrane, the freemembrane surface is offset a certain distance down into chamber 10whereby an electrolyte reservoir is formed between the membrane and lid8. If a wet calibration process is to be performed, said reservoirbetween the membrane and the barrier layer contains, as before, asuitable wetting agent, possibly the same electrolyte liquid.

In this connection it is noted, that, as regards calibration devices forwet gas calibrations and incorporating a membrane, it might be expedientduring manufacture of said devices, to ensure the presence of ahomogenously distributed layer of wetting liquid between the membraneand the barrier layer. This affords the best possible wettingconditions, i.a. due to the fact that the liquid then contacts the drygas immediately upon breaking of the barrier layer.

In the embodiment shown in FIG. 8, housing 2 is provided with asupplementary shoulder 22 between the opening of housing 2 and shoulder6. Shoulder 22 serves to increase the distance which the cutting edge ofthe membrane ring travels relative to barrier layer 3 during rupture ofsaid layer. This is of importance in relation to the extent to which thepartially cut off barrier layer "lid" will fold away from the measuringsurface of the sensor. For the sake of clarity both the membrane ringand the membrane are omitted in FIG. 8.

The effect is obtained by arranging barrier layer 3 as shown in FIG. 8.As appears a part of layer 3--seen in cross section--forms a hypotenusein a triangle the other two sides of which are formed by the shoulder 22and the inner side of housing 2, respectively. When the sensor--asindicated by line 24--is inserted into the housing in the direction ofarrow 23 the barrier layer will be forced in axial and radial directionstowards the above-mentioned two sides of the triangle, this resulting inthat the barrier layer being retracted in the direction of arrow 25. Theattained effect is primarily important in connection with the use of asensor whose forward directed measuring surface only projectsinsignificantly "above" the rest of the forward part of the sensor.Nothing prevents, however, this technique from being applied inconnection with other embodiments of the calibration device.

One could also envisage the above described effect to be obtained in adifferent way, e.g. by increasing the axial length of the portion of thehousing extending between shoulder 6 and the edge of the opening of thehousing. This--together with a complementary formed, annular, axiallydirected recess in the sensor--gives the same effect.

To visualize the order of magnitude of the size of the calibrationdevice the following dimensions of the calibration device as shown inFIG. 3 can be stated: The axial height ca. 10 mm, the diameter ofcalibration chamber 4 ca. 10 mm and the diameter of second chamber 10ca. 12 mm. There is a distance of ca. 4 mm between shoulder 6 and theinner side of lid 8, and the distance between barrier layer 3 andshoulder is ca. 2 mm.

FIG. 9 shows a perspective view of an alternative embodiment of amembrane ring. The membrane ring 30--which in real life will have asomewhat larger d:h ratio than the one illustrated--is a two-piececonstruction consisting of a slotted squeezing ring 31 and a relativelyrigid tightening ring 32 arranged about the squeezing ring. Both saidrings are preferably made from a plastic e.g. POM (polyacetal).Tightening ring 32 has on its inner side facing the squeezing ring anannular projection 33 abutting the outer side of the squeezing ring. Thetightening ring is slidable in the axial direction along the outer sideof the squeezing ring, as will be explained in more detail later.

Squeezing ring 31 comprises a by and large circular cylindrical section34 which smoothly passes into a conical, diverging section 35. As shownin the figure conical section 35 comprises a plurality of spring arms 36arranged equidistantly along the circumference. Spring arms 36 branchesoff from cylindrical section 34, and due to this one-sided attachmentthey are able to move resiliently to and fro in the radial direction.When unbiased the arms take up the slightly outwardly sloping positionsshown in FIG. 9.

Each spring arm 36 has on the outer side near its distal end a recess38, and has on its inner side an inwardly directed projection 40. Whenspring arms 36 are forced radially inwards towards the center line ofthe squeezing ring, a by and large continuous, annular groove will beestablished on the outer side of the squeezing ring while on the innerside of said ring a by and large continuous, annular projection will beformed.

On the inner side of squeezing ring 31 there is an annular breast 41 atthe bases of spring arms 36. The outwardly directed slopes of springarms 36 are adjusted in such a way to the radial extension ofprojections 40 that an axis parallel projection of the innermost edge ofsaid projections 40 approximately will coincide with the transition zonebetween breast 41 and spring arms 36.

Membrane ring 30 further comprises a membrane which is to be attached ina taut, stretched-out state across the measuring surface of the sensorconcurrent with the fastening of the membrane ring to the sensor. Thismembrane--which is omitted in FIG. 9 for the sake of clarity, butappears in FIGS. 10 and 11--is initially fastened across the centralaperture in the cylindrical end of the squeezing ring. This has beenobtained by wrapping the membrane around the entire outercircumferential edge of the squeezing ring, the peripheral membrane partbeing held between said ring 31 and tightening ring 32.

FIGS. 10 and 11 show schematically how the two-piece membrane ring 30cooperates with the calibration device 1 and a sensor 42 during acombined calibration and membrane replacement procedure.

FIG. 10 depicts calibration device 1, membrane ring 30 and sensor 42prior to a combined calibration and membrane replacement procedure. Thelid of the calibration device has been removed, and sensor 42 has beenpartly inserted with its measuring surface ahead into the second chamber10 by moving the sensor in the direction indicated by an arrow 44. Asalso shown, second chamber 10 contains a suitable amount of wettingagent/electrolyte 11. As previously mentioned membrane 12 proper is heldbetween squeezing ring 31 and tightening ring 32.

In the situation shown sensor 42, which here possesses an annularshoulder 45, has been inserted into the calibration device to such anextent, that the front side of said shoulder just contacts breast 41 onsqueezing ring 31. As appearing from FIG. 10 this insertion is possiblebecause spring arms 36 in their unbiased rest positions are slopingoutwards to the degree necessary to allow shoulder 45 to passprojections 40 on the separate spring arms. In the situation shown,barrier layer 3 has not yet been ruptured, and thus the calibrationfluid in calibration chamber 4, e.g. a gas, is not in contact with themeasuring surface of the sensor.

Upon further insertion of sensor 42 into the calibration device thesituation appearing from FIG. 11 is reached. As shown in this figurebarrier layer 3 has now been ruptured and membrane ring 30 has beenattached to sensor 42 in such a way that membrane 12 is held in a taut,stretched-out state across the measuring surface of said sensor.

The fastening of the membrane ring to the sensor has been effected inthe following way: After the under side of tightening ring 32 hasruptured barrier layer 3--in the guided manner previouslydescribed--tightening ring 32 abuts shoulder 6 of the calibrationdevice. As a result of further downwards advancement of the sensor,tightening ring 32 has been displaced upwards along the outer side ofthe squeezing ring until engagement with groove(s) 38 near the distalends of the spring arms. During this displacement of tightening ring 32the spring arms have gradually been forced radially inwards with theresult that their projections 40 have entered into engagement behindshoulder 45 on the sensor. The axial distance between projections 40 andbreast 41 is adapted in such a manner to the axial height of shoulder 45that membrane ring 30 be fastened axially indisplaceable to sensor 42.The engagement between tightening ring 32 and the locking groove 38entails that the engagement between the membrane ring and the sensor ismaintained after the calibration device has been removed from the sensorupon completed calibration. At the same time the relative displacementbetween tightening ring 32 and squeezing ring 31 has effected atautening and stretching-out of membrane 12. Consequently, the system isnow ready for calibration.

It is noted that in this state tightening ring 32 is held firmly betweensensor 42 and shoulder 6. This entails partly that the sensor isprevented from being inserted further into the calibration device,partly that calibration chamber 4 is sealed against the surroundingsduring calibration. The mutual engagement between tightening ring 32 andsensor 42 will be maintained after separating the sensor and thecalibration device, meaning that no unintended interaction can occurbetween the surroundings and the volume of electrolyte trapped betweenthe sensor and the membrane. Upon completed calibration the sensor isseparated from the calibration device and is now ready for use.

When, at a later time, it is desired to remove the membrane ring and themembrane from the sensor--e.g. in connection with a newcalibration--this is done simply by displacing tightening ring 32 in thedirection opposite the direction of the initial displacement, i.e. inthe direction towards the cylindrical section 34 of squeezing ring 31.As the tightening ring approaches this section, spring arms 36 graduallyreturn to their initial, slightly outwardly sloping rest positionsmeaning that projections 40 disengage breast 45 of sensor 42. Eventuallymembrane ring 30 as a whole will be released from the sensor which isthen ready for a new calibration procedure etc.

We claim:
 1. A process for calibrating a sensor in a calibration fluid, said process comprising:(a) holding a calibration fluid of a known composition or condition in a calibration chamber of a calibration housing, said calibration housing being adapted for engagement with said sensor for providing a releasable fluid-tight seal between said calibration housing and at least part of said sensor, said calibration chamber comprising a rupturable barrier on at least one side for enabling said calibration chamber to remain fluid-tight with said calibration fluid sealed therein prior to calibrating said sensor; (b) engaging said sensor with said calibration chamber at said rupturable barrier by applying said sensor against said barrier with sufficient force to rupture said barrier, thereby producing an opening in said barrier; (c) moving at least part of said sensor through said opening and into said calibration chamber so that at least part of said sensor contacts said calibration fluid therein, said sensor and said calibration housing engaging for providing a releasable fluid-tight seal of at least part of said sensor in said calibration chamber.
 2. A process as defined in claim 1, wherein said calibration chamber of said calibration housing is a first fluid-tight chamber, said calibration housing further comprises a second fluid-tight chamber adjacent to said first fluid-tight calibration chamber containing a fluid having a second predetermined composition or condition, said second chamber being separated from said first chamber by said rupturable barrier, and being covered on at least one side by a lid for making said second chamber fluid-tight;and further comprising the steps of: (d) opening said lid of said second chamber; (e) inserting said sensor into and through said second chamber and rupturing said rupturable barrier for bringing said calibration fluid and said second fluid into mixing contact, said sensor and said calibration housing engaging for providing a releasable fluid-tight seal of at least part of said sensor in said calibration chamber.
 3. A process as defined in claim 1 further comprising the steps of:(a) holding a calibration fluid of a known composition or condition in a fluid-tight calibration chamber of a calibration housing, said chamber comprising a rupturable barrier on at least one side; (b) applying the sensor against the barrier with sufficient force to rupture the barrier, thereby producing an opening in the barrier; (c) moving at least part of the sensor through the opening and into the chamber so that at least part of the sensor contacts the calibration fluid therein; (d) measuring the composition or the condition of the fluid with the sensor to obtain a corresponding measurement value; and (e) calibrating the sensor so that the measurement value is approximately equal to the known composition or condition of the fluid.
 4. A process as defined in claim 3, wherein the calibrating step is performed while at least part of the sensor is in the chamber.
 5. A device for calibrating a sensor, said device comprising;(a) a fluid-tight calibration chamber of a calibration housing, said chamber comprising a rupturable barrier on at least one side; (b) a calibration fluid inside the chamber, said fluid having a predetermined composition or condition, wherein said barrier is configured so that the sensor makes contact with the fluid when the barrier is ruptured by the sensor; and (c) a membrane configured essentially parallel to the barrier and outside of the first chamber, the membrane being permeable to components to which the sensor is sensitive, and the device further comprising a fixing means for cooperating with the membrane and the sensor to secure the membrane to the sensor in a stretched state when the sensor is essentially perpendicularly applied against the membrane.
 6. A device as defined in claim 5, wherein the fixing means comprises an elastically deformable fixing ring secured to the side of the membrane facing away from the barrier layer, the fixing ring being configured to engage the sensor when the sensor is inserted into the fixing ring.
 7. A device as defined in claim 5, wherein the fixing means comprises an elastically deformable fixing ring configured between the barrier and the membrane, the fixing ring being configured to engage the sensor when the sensor is inserted into the fixing ring.
 8. A device as defined in claim 5, wherein the fixing means comprises a membrane ring comprising a tubular squeezing ring and a tightening ring, the latter being arranged radially about the former in such a way as to be axially displaceable along the outer side of the squeezing ring, the tightening ring being configured to maintain the membrane in a stretched state across the central aperture in the end of the squeezing ring facing the barrier, and the squeezing ring possessing a plurality of axially extending spring arms the distal ends of which are resiliently movable between a locking position and a rest position, wherein the spring arms are configured in slightly outwardly sloping positions as compared to the locking position, and the spring arms on their outer sides have locking means configured to engage with the tightening ring thereby locking the spring arms in the locking position, and the spring arms on their inner sides have engaging means which in the locking position of the arms are configured to engage with the sensor thereby attaching the membrane ring axially indisplaceable to the sensor.
 9. A device as defined in claim 8, wherein the locking means on the spring arms are configured adjacent to the distal ends of the spring arms.
 10. A device for calibrating in a calibration fluid, said device comprising a calibraton housing with at least two chambers, wherein(a) a first fluid-tight chamber forms a calibration chamber holding a first fluid of a predetermined composition or condition, said first chamber having an opening sealed by a fluid-tight rupturable barrier; (b) a second fluid-tight chamber is configured adjacent to said first fluid-tight chamber containing a second fluid of a predetermined composition or condition, said second chamber being separated from said first chamber by said rupturable barrier and being sealingly covered by a lid; (c) said calibration housing is configured for engagement with said sensor for providing a releasable fluid-tight seal between at least part of said sensor and said first chamber when the sensor is inserted in said calibration housing through said second chamber and said barrier and into said first chamber.
 11. A device as defined in claim 10, wherein barrier comprises a non-elastic material.
 12. A device as defined in claim 10, wherein the first fluid comprises a dry gas with a known content of components to which the sensor is sensitive.
 13. A device as defined in claim 12, wherein the dry gas is selected from the group consisting of O₂, CO₂, and combinations thereof.
 14. A device as defined in claim 10, wherein the first and second chambers are configured so that the second fluid contacts the first fluid when the barrier is ruptured.
 15. A device as defined in claim 14, wherein the first fluid is a dry gas and the second fluid is a liquid.
 16. A device as defined in claim 15, wherein the second fluid contains bicarbonates.
 17. A device as defined in claim 15, wherein the second fluid comprises a liquid containing a gas selected from the group consisting of:(i) CO₂ in an amount corresponding to a partial gas pressure of carbon dioxide in the range of 40-80 mm Hg; (ii) O₂ in an amount corresponding to a partial gas pressure in the range of 40-170 mm Hg; and (iii) combinations of the foregoing.
 18. A device that is suitable for use in the calibration of a sensor, said device comprising a sealed housing comprising at least two chambers, wherein(a) a first fluid-tight calibration chamber is covered on at least one side by a fluid-tight, rupturable barrier, and said first chamber contains a first fluid having a predetermined composition or condition; (b) a second chamber is configured adjacent to the first chamber and is separated from the first chamber by the fluid-tight, rupturable barrier, and said second chamber contains a second fluid and is covered on at least one side by a lid; and (c) a membrane is configured essentially parallel to the barrier and outside of the first chamber, the membrane being permeable to components to which the sensor is sensitive, and the device further comprising a fixing means configured to cooperate with the membrane and the sensor in such a way that the membrane is secured to the sensor in a stretched state when the sensor is essentially perpendicularly applied against the membrane.
 19. A device as defined in claim 18, wherein the fixing means comprises an elastically deformable fixing ring secured to the side of the membrane facing away from the barrier layer, the fixing ring being configured to engage the sensor when the sensor is inserted into the fixing ring.
 20. A device as defined in claim 18, wherein the fixing means comprises an elastically deformable fixing ring configured between the barrier and the membrane, the fixing ring being configured to engage the sensor when the sensor is inserted into the fixing ring.
 21. A device as defined in claim 18, wherein the fixing means comprises a membrane ring comprising a tubular squeezing ring and a tightening ring, the latter being arranged radially about the former in such a way as to be axially displaceable along the outer side of the squeezing ring, the tightening ring being configured to of maintain the membrane in a stretched state across the central aperture in the end of the squeezing ring facing the barrier, and the squeezing ring possessing a plurality of axially extending spring arms the distal ends of which are resiliently movable between a locking position and a rest position, wherein the spring arms are configured in slightly outwardly sloping positions as compared to the locking position, and the spring arms on their outer sides have locking means configured to engage with the tightening ring thereby locking the spring arms in the locking position, and the spring arms on their inner sides have engaging means which in the locking position of the arms are configured to engage with the sensor thereby attaching the membrane ring axially indisplaceable to the sensor.
 22. A device as defined in claim 21, wherein the locking means on the spring arms are configured adjacent to the distal ends of the spring arms.
 23. A device for calibrating a sensor in a calibration fluid, said device comprising a calibration housing with a calibration chamber holding a calibration fluid of a known composition or condition, whereinsaid calibration chamber has an opening for allowing said sensor to be at least partially inserted in said calibration chamber to enable said sensor to at least partially contact said calibration fluid during calibration of said sensor; said opening is sealed by a fluid-tight rupturable barrier for enabling said calibration chamber to remain fluid-tight with said calibration fluid sealed therein prior to calibrating said sensor; said barrier is rupturable by said sensor when at least part of said sensor is moved through said opening and into said calibration chamber and into contact with said calibration fluid; and said calibration housing is configured for engagement with said sensor for providing a releasable fluid-tight seal between at least part of said sensor and said calibration chamber when the sensor is inserted in said calibration housing.
 24. A device as defined in claim 23 wherein said calibration housing comprises a rim extending around said opening, said rim cooperating with said sensor provide said releasable fluid-tight seal.
 25. A device as defined in claim 23, wherein said device further comprises:means for limiting the depth to which the sensor is capable of entering the housing.
 26. A device as defined in claim 23, wherein the fluid comprises a dry gas with a known content of components to which the sensor is sensitive.
 27. A device as defined in claim 26, wherein the dry gas is selected from the group consisting of O₂, CO₂, and combinations thereof.
 28. A device as defined in claim 23, wherein the barrier of the calibration chamber comprises a non-elastic material.
 29. A device as defined in claim 28, wherein the barrier comprises aluminum foil of a thickness of about 0.02-0.04 mm and the aluminum foil has a polymeric foil coating on at least one side. 